"The truth is that this is the first post of the Valve Linux blog. This blog is where you can find the latest information from Valve about our Linux development efforts. Avoid the rumors and speculations that multiply on the Web. Instead, come to the source - a blog where people who are interested in Linux and open source game development can get the latest information on Valve's efforts in this arena. In this initial post, we'll introduce the team (and a bit of its history) and then give you a snapshot of what we're currently doing." Steam, Source, and Left 4 Dead 2 coming to Linux. We know why.

The thing is that while wine is a good solution for playing windows games on x86/x86_64 linux. It is not useful for other architectures.
As there are some rumours going around about Valve wanting to build some kind of linux based gaming device (console) they would profit from porting to linux instead of using wine. As wine does not work on other architectures, but when something is ported correctly it should be trivial to recompile for other architectures (arm anyone?).